1. Field of the Invention
This invention relates to a device of the type including a converter for converting a main supply voltage into a positive and a negative buffer voltage, an inverter for converting the buffer voltages into a control-voltage to be supplied to at least one load connected to the device, for example, an electric motor, and a control-device for controlling the frequency of the control-voltage, wherein the inverter includes for both the positive and the negative buffer voltages a main thyristor energizable by the control-device as well as a commutation circuit for each of the main thyristors energizable by the control-device.
2. Description of the Prior Art
In a known device of the above-noted type, an auxiliary thyristor of the corresponding commutation circuit has to be triggered for switching off a main thyristor, as a result of which an oscillating current generated by reactive circuit elements starts flowing through the commutation circuit. In the first phase of the oscillating current the auxiliary thyristor is switched off, whereas in the subsequent phase of the oscillating current the current flowing through the main thyristor decreases and hence the main thyristor switches off, since the increasing current is conducted away by means of the commutation circuit. Since the current to be conducted away may assume high values, the reactive components of the commutation circuit should be proportioned accordingly, which is disadvantageous in many respects because in the first place the bulk of the reactive components is larger, for example, with regard to the coil included in the commutation circuit and secondly higher losses are involved, which results in the generation of more heat. A further detriment resulting from such disadvantages resides in that due to the generation of heat, for example, in the coil, the characteristics of this coil are adversely affected so that under given conditions a kind of avalanche effect may occur, as a result of which the entire commutation circuit becomes inoperative.
A further disadvantage is that thyristors having to pass a high current have a comparatively long response time. This is particularly important for the switching behavior of the auxiliary thyristor in the commutation circuit.